1254-NumberOfClosedIslands.java

class Solution {
    int res = 0;
    int[][] directions = { { -1, 0 }, { 1, 0 }, { 0, 1 }, { 0, -1 } };

    public int closedIsland(int[][] grid) {
        int m = grid.length;
        int n = grid[0].length;

        for (int i = 0; i < m; i++) {
            if (grid[i][0] == 0)
                dfs(grid, i, 0);
            if (grid[i][n - 1] == 0)
                dfs(grid, i, n - 1);
        }

        for (int j = 0; j < n; j++) {
            if (grid[0][j] == 0)
                dfs(grid, 0, j);
            if (grid[m - 1][j] == 0)
                dfs(grid, m - 1, j);
        }

        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 0) {
                    res += 1;
                    dfs(grid, i, j);
                }
            }
        }
        return res;
    }

    private void dfs(int[][] grid, int i, int j) {
        if (i < 0 || j < 0 || i >= grid.length || j >= grid[0].length || grid[i][j] != 0)
            return;

        grid[i][j] = 1;
        for (int[] d : directions) {
            dfs(grid, i + d[0], j + d[1]);
        }
    }
}